1. Field of the Invention
The present invention relates to a positive resist composition and a method of forming a resist pattern using the positive resist composition.
Priority is claimed on Japanese Patent Application No. 2009-055745, filed Mar. 9, 2009, the content of which is incorporated herein by reference.
2. Description of the Related Art
In lithography techniques, for example, a resist film composed of a resist material is formed on a substrate, and the resist film is subjected to selective exposure of radial rays such as light or electron beam through a mask having a predetermined pattern, followed by development, thereby forming a resist pattern having a predetermined shape on the resist film.
As shortening of the wavelength of the exposure light source progresses, it is required to improve various lithography properties of the resist material, such as the sensitivity to the exposure light source and the resolution capable of reproducing patterns of minute dimensions. As a resist material which satisfies these conditions, a chemically amplified resist is used, which includes a base resin that exhibits a changed solubility in an alkali developing solution under action of acid and an acid generator that generates acid upon exposure.
Currently, resins that contain structural units derived from (meth)acrylate esters within the main chain (acrylic resins) are now widely used as base resins for resists that use ArF excimer laser lithography, as they exhibit excellent transparency in the vicinity of 193 nm.
Here, the term “(meth)acrylic acid” is a generic term that includes either or both of acrylic acid having a hydrogen atom bonded to the α-position and methacrylic acid having a methyl group bonded to the α-position. The term “(meth)acrylate ester” is a generic term that includes either or both of the acrylate ester having a hydrogen atom bonded to the α-position and the methacrylate ester having a methyl group bonded to the α-position. The term “(meth)acrylate” is a generic term that includes either or both of the acrylate having a hydrogen atom bonded to the α-position and the methacrylate having a methyl group bonded to the α-position.
As a technique for further improving the resolution, a lithography method called liquid immersion lithography (hereafter, frequently referred to as “immersion exposure”) is known in which exposure (immersion exposure) is conducted in a state where the region between the objective lens of the exposure apparatus and the sample is filled with a solvent (an immersion medium) that has a larger refractive index than the refractive index of air (see, for example, Non-Patent Document 1).
According to this type of immersion exposure, it is considered that higher resolutions equivalent to those obtained using a shorter wavelength light source or a larger NA (numerical aperture) lens can be obtained using the same exposure light source wavelength, with no lowering of the depth of focus. Furthermore, immersion exposure can be conducted using a conventional exposure apparatus. As a result, it is expected that immersion exposure will enable the formation of resist patterns of higher resolution and superior depth of focus at lower costs. Accordingly, in the production of semiconductor devices, which requires enormous capital investment, immersion exposure is attracting considerable attention as a method that offers significant potential to the semiconductor industry, both in terms of cost and in terms of lithography properties such as resolution.
Immersion lithography is effective in forming patterns having various shapes. Further, immersion exposure is expected to be capable of being used in combination with currently studied super-resolution techniques, such as phase shift methods and modified illumination methods. Currently, as the immersion exposure technique, techniques using an ArF excimer laser as an exposure source are being actively studied. Further, water is mainly used as the immersion medium.
In recent years, fluorine-containing compounds have been attracting attention for their properties such as water repellency and transparency, and active research and development of fluorine-containing compounds have been conducted in various fields.
For example, in the fields of resist materials, currently, an acid-labile group such as a methoxymethyl group, tert-butyl group or tert-butyloxycarbonyl group is being introduced into a fluorine-containing polymeric compound, and the fluorine-containing polymeric compound is then used as a base resin for a chemically amplified positive resist.
However, when such a fluorine-containing polymeric compound is used as a base resin for a chemically amplified positive resist, disadvantages are caused in that a large amount of an out gas is generated following exposure, and the resistance to a dry etching gas (namely, the etching resistance) is unsatisfactory.
Recently, as a fluorine-containing polymeric compound exhibiting excellent etching resistance, a fluorine-containing polymeric compound having an acid-labile group containing a cyclic hydrocarbon group has been reported (see, for example, Non-Patent Document 2).
In addition, a fluorine-containing polymeric compound has been reported in order to provide a resist film with water repellency in a resist composition for immersion exposure (see, for example, Non-Patent Document 3).
[Non-Patent Document 1] Proceedings of SPIE (U.S.), vol. 5754, pp. 119-128 (2005)
[Non-Patent Document 2] Proceedings of SPIE (U.S.), vol. 4690, pp. 76-83 (2002)
[Non-Patent Document 3] Journal of Photopolymer Science and Technology, vol. 19, No. 4, pp. 565-568 (2006)
In immersion exposure, a resist material is required which exhibits not only general lithography properties (e.g., sensitivity, resolution, etching resistance and the like), but also properties suited for immersion lithography.
For example, in immersion exposure, when the resist film comes into contact with the immersion medium, elution of a substance contained in the resist film into the immersion medium (substance elution) occurs. This elution of a substance causes phenomena such as degeneration of the resist film and change in the refractive index of the immersion medium, thereby adversely affecting the lithography properties. The amount of the eluted substance is affected by the properties of the resist film surface (e.g., hydrophilicity, hydrophobicity, and the like). For example, by enhancing the hydrophobicity of the resist film surface, the elution of a substance can be reduced.
Further, when the immersion medium is water, and immersion exposure is performed using a scanning-type immersion exposure apparatus as disclosed in Non-Patent Document 1, a water tracking ability in which the immersion medium is capable of tracking the movement of the lens is required. When the water tracking ability is poor, the exposure speed becomes low, and as a result, there is a possibility that the productivity may be adversely affected. It is presumed that the water tracking ability can be improved by enhancing the hydrophobicity of the resist film surface (rendering the resist film surface hydrophobic). It has been desired to achieve high speed scanning by improving the water tracking ability.
However, the effect of enhancing the hydrophobicity of the resist film surface is not yet satisfactory in the conventional resist compositions containing a fluorine-containing compound.
The present invention takes the above circumstances into consideration, with an object of providing a positive resist composition which is capable of forming a resist film having a highly hydrophobic surface, and a method of forming a resist pattern using the positive resist composition.